After a four and a half day hiatus, Tropical Storm Gabrielle began to regenerate south of Bermuda on 10 September 2013. AWIPS images of 1-km resolution POES AVHRR 0.86 Âµm visible channel and 12.0 Âµm IR channel data at 13:41 UTC (above) showed overlays of surface buoys, surface analysis, and ASCAT surface scatterometer winds. A number of these ASCAT winds exhibited speed values of 45-46 knots, prompting NHC to adjust the intensity of Gabrielle upward:

TROPICAL STORM GABRIELLE SPECIAL DISCUSSION NUMBER 9
NWS NATIONAL HURRICANE CENTER MIAMI FL AL072013
130 PM AST TUE SEP 10 2013
A PAIR OF ASCAT PASSES FROM 1342Z AND 1436Z INDICATE THAT GABRIELLE
IS STRONGER THAN PREVIOUSLY ESTIMATED. BOTH PASSES SHOWED SEVERAL
45-KT WIND RETRIEVALS...AND THAT IS THE INITIAL INTENSITY FOR THIS
SPECIAL ADVISORY. THE INTENSITY FORECAST HAS BEEN ADJUSTED UPWARD
AT 12 AND 24 HOURS TO 50 KT BASED ON THE INCREASE IN THE INITIAL
INTENSITY. THE ANALYZED 34-KT WIND RADII HAVE ALSO BEEN ADJUSTED
OUTWARD BASED ON THE ASCAT DATA...AND THEIR FORECAST HAS BEEN
MODIFIED AS WELL TO REFLECT THE LARGER WIND FIELD. AN AIR FORCE
RESERVE AIRCRAFT WILL INVESTIGATE GABRIELLE LATER THIS AFTERNOON TO
PROVIDE MORE DETAILED INFORMATION ON THE STRENGTH AND STRUCTURE OF
THE CYCLONE.

A sequence of 6-hour interval 850 hPa relative vorticity product images from the CIMSS Tropical Cyclones site (above; click image to play animation) showed that the lower-tropospheric vorticity feature associated with Tropical Storm Gabrielle remained fairly intact during the 06-10 September period (between the times when Gabrielle had exhibited tropical cyclone organization and intensity).

McIDAS images of 1-km resolution GOES-13 0.63 Âµm visible channel data (above; click image to play animation) revealed a well-defined central dense overcast (CDO) convective burst early in the day, followed by the emergence of the low-level circulation of Gabrielle as the middle and high-altitude cloud layers were sheared off to the northeast.

GOES-15, in service as GOES-West over the Equator at 135 W, has experienced navigation anomalies on each of the past three mornings, September 8, 9 and 10, 2013 (as noted here, for example). Navigation anomalies occur approximately between 0830 and 1100 UTC. GOES Engineers are investigating the source of the anomaly, which can be on the order of 40 km. The animation above is the shortwave infrared from Tuesday 10 September. Animations for 6, 7, 8 and 9 September are below. Note that even the 6 September and 7 September have small navigation issues.

(Update: From SATOPS: NOTE: GOES-15 (West) Image registration (alignment of images to earth grid coordinates) was shifted by approximately 40 kilometers on DOY251 (ed. note: 8 September) from 08:50z to 10:50z and shifted by approximately 25 kilometers on DOY252 & DOY253 (ed. note: 9 and 10 September) from 9:30z to 10:40z. The earth coordinate grid shift was due to a lack of available valid star data for predicting the proper attitude pointing profile around eclipse. The root cause is under investigation.) In other words, the Star Pointer was unable to find enough targets to produce accurate navigation. (Link)

Update, 11 September: The anomalies continued on 11 September, starting after the 0845 UTC image, peaking at the 1000 UTC image, and subsiding by 1045 UTC. Link.

Update, 17 September: Large navigation anomalies were not present overnight.

The Morgan Fire began burning in the Mount Diablo State Park northeast of San Francisco during the afternoon hours on 08 September 2013, which caused the evacuation of dozens of homes near the fire. McIDAS images of 4-km resolution GOES-15 3.9 Âµm shortwave IR data (above; click image to play animation; also available as a .mp4 file) revealed that the fire “hot spot” (red color enhancement) first appeared on the 21:30 UTC (2:30 PM local time) image. The fire appeared to “settle down” for a few hours after sunset, but then there was an apparent flare-up of the fire from 04:00 – 06:30 UTC.

One thing to note on the GOES-15 shortwave IR animation above is that there was a significant Image Navigation and Registration (INR) anomaly during the 08:45 – 10:41 UTC period. An AWIPS 1-km resolution Suomi NPP VIIRS 3.74 Âµm shortwave IR image was available at 09:24 UTC (during the GOES-15 INR anomaly) — a comparison of this VIIRS image with the closest available GOES-15 shortwave IR image (below) demonstrates the value of higher spatial resolution VIIRS data to aid in a more accurate determination of the actual fire size and location.

Suomi NPP VIIRS 3.74 Âµm and GOES-15 3.9 Âµm shortwave IR images

Hot spots associated with the ongoing Rim Fire — so far the 3rd-largest fire on record in California — can also be seen about 100 miles to the east-southeast.

1-km resolution GOES-13 0.63 Âµm visible channel images (above; click image to play animation) revealed a pronounced convective burst associated with Tropical Depression #7, which was centered just southeast of Puerto Rico late in the day on 04 September 2013. On the final visible image before darkness, a pair of small but distinct overshooting tops could be seen, which actually signalled the beginning of another convective burst.

The corresponding 4-km resolution GOES-13 10.7 Âµm IR channel images (below; click image to play animation) showed that there were a series 3 well-defined convective bursts (2 more followed after darkness), each exhibiting areas with cloud-top IR brightness temperatures of -80Âº C or colder (violet color enhancement). On the final image in the animation (at 02:45 UTC), the coldest IR brightness temperature was -86Âº C This was about the time that Tropical Depression #7 was upgraded to Tropical Storm Gabrielle.

Two GOES-13 IR images at 03:45 UTC on 05 September from the CIMSS Tropical Cyclones site (below) showed that (1) Gabrielle was in an environment characterized by a light amount (around 10 knots) of deep layer (200-850 hPa) wind shear, and (2) a well-defined low-level cyclonic circulation was difficult to distinguish with the ASCAT surface scatterometer winds at that time.

GOES-13 IR image with deep layer wind shear analysis

GOES-13 10.7 Âµm IR image with ASCAT surface scatterometer winds

===== 05 September Update =====

DMSP-17 SSMIS 85 GHz microwave brightness temperature

On the morning of 05 September, a DMSP-17 SSMIS 85 GHz microwave brightness temperature image at 10:42 UTC (above) and GOES-13 10.7 Âµm IR images (below) suggested that the mid-level circulation center of of Gabrielle was reforming well to to the east of the original fix location low-level circulation center. Around the time of the end of the IR image animation, Gabrielle was downgraded to a Tropical Depression.